This invention relates to passive thermal control of spacecraft, and, more particularly, to spacecraft protected by an external coating which aids in thermal control and protects the spacecraft against damage by a flux originating externally.
Spacecraft are subjected to a wide range of thermal environments during service. One side of the spacecraft may face away from the sun into the void of free space, while the other side faces the sun. Heat is radiated into free space from the side of the spacecraft facing away from the sun to cool the spacecraft, but the side of the spacecraft facing the sun is heated intensively by direct sunlight.
Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons, electronic devices, and/or sensitive instruments, within acceptable operating limits. Active temperature control usually involves mechanical or electrical devices, such as heat pipes or electrical heaters. The present invention deals with an approach that incorporates a basic passive temperature control technique, but which may be used in an active control mode as well.
One approach to passive temperature control uses surface coatings, sometimes termed xe2x80x9cpaintsxe2x80x9d, on the external surface of the spacecraft. A white coating, for example, has a low solar absorptance, while a black coating has a high solar absorptance. The selective application of such coatings to various elements of the spacecraft exterior greatly aids in controlling their temperatures. The present invention deals with a coating that- is useful in spacecraft temperature control applications.
In most cases, the coating desirably also provides electrical protection to the spacecraft, in addition to providing passive thermal control. A spacecraft is sometimes subjected to electronic charging induced by a flux of electrons originating from an external source. In one example of extreme charging, a solar storm may eject a high flux of electrons from the sun. When the electron flux reaches the spacecraft, it subjects the surface of the spacecraft to a large flux of electrons. These electrons can accumulate as a static charge and eventually produce arcing (i.e., a dielectric breakdown and electrostatic discharge) at the surface of the spacecraft, which may structurally damage the spacecraft and/or interfere with sensitive electronic equipment on or in the spacecraft.
Several passive coating-based approaches are known to protect spacecraft from this type of electrical damage. In one approach a multilayer coating is provided, wherein a top coating serves the thermal control function and an underlying layer is electrically conductive to dissipate electrical charge. Such multilayer coatings are heavy and are difficult to apply because the layers must be quite precisely deposited. Single-layer electrostatic-dissipative paints are also known for spacecraft use. One such white paint, based upon the aluminum-doped zinc oxide pigment of the type disclosed in U.S. Pat. No. 5,094,693, typically has a solar absorptance of from about 0.18 to about 0.22. The white paint described in U.S. Pat. No. 5,820,669 improves upon this performance by providing a solar absorptance of less than 0.1. These paints provide excellent performance in a number of applications. However, the paints described in the ""693 patent and the ""669 patent are limited as to the maximum flux of electrons that may be dissipated because of the maximum electrical conductivities that are possible with their formulations, and therefore cannot perform some missions.
There is a need for a further improved thermal-control coating that is operable and stable in a space environment, which has tailorable thermal properties, and which protects the spacecraft against damage by externally induced electronic fluxes of high magnitudes. The present invention fulfills this need, and further provides related advantages.
The present invention provides a spacecraft protected by a coating, and the coating material. The color of the coating may be selected to provide desired thermal properties. The coating is structured to protect the spacecraft against electrical damage produced by the accumulation of electrical charge induced by intensive external fluxes of electrons. As a result of its high index of refraction and resulting excellent hiding power, the coating may be applied in thinner coatings than conventional protective coatings, reducing the weight and cost of the spacecraft. The coating pigment of the invention may be mixed with other pigments to optimize performance for a wide variety of conditions. The coating may be used in either a passive or an active mode.
In accordance with the invention, a spacecraft protected by a pyroelectric/ferroelectric coating comprises a spacecraft having an external surface, and a coating on the external surface of the spacecraft. The coating includes a binder, and a plurality of pyroelectric/ferroelectric pigment particles bound together by the binder. The pyroelectric/ferroelectric pigment particles are preferably ferroelectric pigment particles, each of which comprises a ferroelectric pigment material having a ferroelectric/paraelectric transition.
In another embodiment, each pyroelectric/ferroelectric pigment particle may be described as comprising a ferroelectric pigment material having a dielectric permittivity exceeding about 200, typically from about 200 to about 25,000, and an electronic band gap exceeding about 2.5 electron volts. In yet another embodiment, each pyroelectric/ferroelectric pigment particle may be described as comprising a ferroelectric pigment material which stores electronic charge on the particle surface when exposed to an electron flux, and which thereafter releases the stored electronic charge over a period of time.
The present invention is a complete departure from the approaches of the prior art to multi-layer and single-layer paints and coatings which protect spacecraft from electronic charge accumulations. Only a single layer is used, avoiding the application difficulties experienced with multi-layer paints. Previously, the single-layer paints had been described as electrostatically dissipative (ESD), and the design approach was based upon obtaining a sufficiently high electrical conductivity of the paint while retaining the desired thermal properties. The high electrical conductivity dissipates the electronic charge as it builds up, and eventually conducts the charge to ground. The success of this design approach in protecting the spacecraft is based upon achieving a sufficiently high electrical conductivity in the paint. This approach works well for many spacecraft applications, but is limited in its ability to protect the spacecraft against very high electronic fluxes because there are physical limits on the ability to increase the electrical conductivity of the paint while retaining desired thermal properties.
In the present approach, the accumulation of charge on the surface of the particles and the coating is acceptable, as long as that accumulation of charge does not produce a high surface voltage that could lead to arcing (that is, a dielectric breakdown and electrostatic discharge) or other type of electrical damage. The coating absorbs and stores the electronic charge at the surface of the coating as it accumulates during a high-flux event while preventing a significant increase in surface voltage, and both simultaneously and thereafter gradually conducts the accumulated charge to ground. The coating is thereby xe2x80x9cresetxe2x80x9d for the next high-flux event. The coating effectively acts as an intentionally leaky thin capacitor applied over a large area of the external surface of the spacecraft. The coating therefore has a small electrical conductivity, expressed as a surface resistivity of less than or equal to about 1010 ohms per square at room temperature. This conductivity is achieved by doping the pigment material to a level that its conductivity is sufficient to provide the required conductivity to the coating. Desirably, the electrical surface resistivity is not less than about 108 ohms per square, inasmuch as the doping required to achieve such low electrical surface resistivity would likely adversely affect the optical properties of the coating.
When a conventional ESD paint is operated on a surface at a low temperature, the electrical surface resistivity of the pigment and thence the paint increases so that a portion of its electrical protective capability is lost. In the coating of the present invention, the electrical surface resistivity of the pigment may change, but the polarization capability of the coating stores the electrical charge to permit a gradual dissipation according to the electrical conductivity that remains. The stored charge does not result in a high surface voltage that leads to arcing, because of the high dielectric constant of the pigment and thence the coating. The present coating thus provides protection against electron fluxes that persists at very low temperatures., an important improvement over available ESD paints.
Materials which exhibit a ferroelectric/paraelectric transition, termed herein and in the art a xe2x80x9cferroelectricxe2x80x9d material, are preferred for use as the pigment in such a coating. Such ferroelectric pigment materials accumulate electronic charge without a substantial increase in surface voltage, because they preferably have a dielectric permittivity exceeding about 200. The ferroelectric pigment materials are doped so that they have sufficient electrical surface conductivity to conduct away the accumulated charge relatively slowly during and after the incidence of the external flux. They preferably have an electronic band gap exceeding about 2.5 electron volts. Ferroelectric pigment materials having the ferroelectric/paraelectric properties may be manufactured in various colors, which may be selected to produce the required thermal properties in the coating. The whiter the ferroelectric pigment material, the lower its solar absorptance.
Stated alternatively, the present approach provides for an accumulation of electrical charge at the surfaces of the ferroelectric pigment particles during a high-flux event, while gradually conducting the charge away both during the high-flux event and after it has ended. The coating is thus xe2x80x9cadaptivexe2x80x9d to the conditions experienced in space. In standard low-flux conditions, any introduced electronic charge is conducted away substantially as it develops. As the coating is subjected to fluxes of increasing electronic flux density, the coating harmlessly stores the charge in excess of that which may be conducted away immediately, and conducts that charge away after the external flux ends. Consequently, there is a recovery time which increases with increasing magnitude and duration of the flux, but the coating of the present invention can protect against higher fluxes than possible with the conventional approach. The conventional approach requires that the electrical charge produced by a high-flux event be conducted away as it is created in order to prevent a high voltage at the surface of the coating. However, physical limits on the doping of conventional pigments prevent such conventional paints from dissipating electrical charge sufficiently rapidly in some high-flux conditions, with the result that there may be excessively high voltages at the surface of the paint and consequent electrical arcs.
The present approach thus provides a coating which protects the surface of a spacecraft against excessive electrical charging over a wide range of conditions of external electron fluxes, while also providing passive thermal control with the possibility of active thermal control. Other features and advantages of the present invention will be apparent from the following more detailed description of the presently preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this presently preferred embodiment.